A lower score on the six-minute walk test (6MWT) correlates with disease severity and mortality in patients with systemic sclerosis (SSc), supporting its validity in assessing the prognosis of these patients, a study reports.
Results also showed that an additional meter walked during the test corresponded with a 1% increase in survival.
The study, “Reproducibility and Utility of the 6-minute Walk Test in Systemic Sclerosis,” was published in The Journal of Rheumatology.
The 6MWT is a safe, noninvasive test used to assess the exercise tolerance of patients diagnosed with chronic respiratory diseases, such as pulmonary fibrosis or pulmonary arterial hypertension, and heart failure. This test has also been increasingly used in patients with pulmonary hypertension associated with connective tissue disease, including SSc.
It measures the maximum distance an individual is able to walk over six minutes on a hard, flat surface. The person can walk at their own pace and can stop and rest as many times as they need.
In 2008, the test was recommended as a parameter in SSc clinical trials, but the number of studies that have actually assessed how 6MWT results correlate with patient’s clinical outcomes is still limited.
Researchers have now conducted an observational study to evaluate the usefulness of the 6MWT in SSc patients.
The study, performed between 2002 and 2008, enrolled 56 SSc patients, 39 (70%) of whom had diffuse cutaneous SSc. Most participants were females (68%), and their mean age was 46. Patients underwent at least two six-minute walk tests within a minimum three-month interval. At each evaluation, researchers also analyzed patients’ clinical data, including body mass index and SSc subtype, among other parameters.
Three participants — 5.4% — had SSc-related pulmonary arterial hypertension confirmed by right heart catheterization — the standard test to diagnose pulmonary hypertension.
At the first clinical visit, 31 patients had a normal six-minute walk distance (6MWD) value — a mean value of 524 meters — while the other 25 patients had abnormal, or lower, 6MWT results — an average of 373 meters.
Compared with SSc patients with normal 6MWD values, those with lower scores on their first clinical evaluation were an average of eight years younger — 49 versus 41 years old — and had significantly worse physical activity.
Additionally, they had higher muscle disease scores, more frequent pain in the joints, tendon friction, and higher impairments in lung function, measured by the forced vital capacity (FVC) and the diffusing capacity (DLCO) tests. FVC measures the amount of air that can be forcibly exhaled from the lungs after taking the deepest breath possible, while DLCO measures the ability of the lungs to transfer oxygen from the inhaled air to the blood.
The results suggest that absolute values of the 6MWD correlate with several clinical and functional outcomes of patients.
By the end of the follow-up period, eight patients had died — seven due to SSc, and one from both esophagus cancer and liver cirrhosis. Six of these deaths occurred among patients with abnormal 6MWD values at first referral.
The researchers therefore suggested that 6MWT results are an independent predictor of mortality.
“The 6MWD absolute value at first referral was an independent predictor for both the overall mortality and SSc-related mortality,” the researchers wrote.
They also found that 1 additional meter on the 6MWT was associated with a 1% improvement in survival.
These results support the use of the 6MWT “as a valid tool to assess functional capacity, with a significant prognostic value at first evaluation but also during follow-up in SSc patients with various degrees of organ involvement,” the team concluded.